/Engine/Source/Runtime/MeshConversion/Public/MeshDescriptionBuilder.h
Set the full tangent space of a vertex instance, this is stored as a vec3 normal, vec3 tangent, and a bitangent sign
void SetInstanceTangentSpace ( const [FVertexInstanceID](API\Runtime\MeshDescription\FVertexInstanceID) & InstanceID, const FVector & Normal, const FVector & Tangent, float Sign )